You should have to inro the nut to remove the driveshaft from the hub to be able to replace the outer boost.
The cv joint is retained on the driveshaft by a circlip that sits in a groove, in theory you I don't see why this couldn't come out if everything else was undone bar the driveshaft nut. But when it's altogether as it should be, it shouldn't be able to come out.
Which one did you pop off? offside/near side/ inner/outer?